Agonist-induced signaling is a primary mechanism regulating adhesiveness of platelets, leukocytes and other cells. However, RBCs are generally considered to be inert to agonist-stimulated enhancement of adhesion; thus signaling in sickle (SS) RBCs by any agonist to directly activate SS RBC adhesiveness is relatively unexplored. Thrombospondin (TSP) is an adhesive protein that is abnormally elevated in the plasma of sickle cell patients. While TSP has known agonist properties towards other cells, it has been proposed in sickle cell disease to function purely as an adhesive molecule, by bridging adherent SS RBCs to the endothelium and subendothelial matrix. Here we establish that SS RBCs, in contrast to normal (AA) RBCs, respond to agonist stimulation by TSP to become significantly more adhesive. We also identify a site within TSP, which when immobilized can support SS RBC adhesion, or when soluble, can activate SS RBC adhesion. We further establish a previously unappreciated role for integrin-associated protein (TAP or CD47) on SS RBCs as both a basal adhesion receptor for immobilized TSP and a signal-transducing receptor in response to soluble TSP, and detect a potential physical difference in TAP on SS RBC that may contribute to the higher adhesion of SS versus AA RBCs. Evidence is also provided for a unique synergy between TAP-mediated signaling and shear stress-induced signaling, involving activation of large G-proteins and tyrosine kinases, to ultimately activate an a4B1 integrin-dependent increase in SS RBC adhesion. We therefore propose to: 1) identify proximal events in the activation of a4B1-mediated SS RBC adhesion, 2) define the basis for the apparent physical difference in sickle cell TAP and ask whether that difference contributes to the enhanced basal and stimulated sickle cell adhesion, 3) delineate the specific signaling pathway(s) induced by stimulation of lAP that results in increased SS RBC adhesiveness, 4) determine the role and mechanism of TAP-mediated SS RBC activation in SS RBC adhesion to endothelial cells, and 5) test the hypothesis that TAP contributes to SS RBC adhesion and pathology in vivo in studies of human sickle cell flow through a rat cremaster muscle model system. These data therefore provide new fundamental models of sickle cell adhesion and identify multiple potential therapeutic targets for down-regulating sickle cell adhesiveness and potentially preventing vaso-occlusive crises.